01 - Preheat oven to 375°F and position the rack in the lower third of the oven. Line a baking sheet with parchment paper and set aside.
02 - In a large mixing bowl, combine the chopped rhubarb, sliced strawberries, blueberries, and raspberries. Add the granulated sugar, brown sugar, cornstarch, cinnamon, vanilla extract, and lemon juice. Toss gently until all the fruit is evenly coated, then set aside to macerate.
03 - Unroll or place the unbaked pie crust into a 9-inch pie dish. Trim any excess dough hanging over the edges and crimp the border decoratively as desired.
04 - Spoon the berry-rhubarb mixture evenly into the prepared pie crust, distributing the fruit in an even layer and pouring any accumulated juices over the top.
05 - In a medium bowl, whisk together the all-purpose flour, rolled oats, brown sugar, salt, and cinnamon. Add the cold cubed butter and use your fingers or a pastry blender to cut it in until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining.
06 - Sprinkle the crumble topping evenly over the fruit filling, covering the surface completely. Use your hands to gently press some of the mixture together to create varied texture.
07 - Place the pie on the prepared parchment-lined baking sheet and bake for 50 to 55 minutes, until the crumble topping is deep golden brown and the filling is visibly bubbling around the edges. If the top begins to brown too quickly, tent loosely with aluminum foil after the first 30 minutes.
08 - Transfer the pie to a wire cooling rack and allow it to cool for at least 2 hours before slicing so the filling has time to set properly. Serve warm or at room temperature, optionally accompanied by vanilla ice cream or freshly whipped cream.